1. Field of the Invention
This invention relates generally to the field of solids handling and more particularly, to methods and apparatus useful for procuring a representative sample of pneumatically conveyed solid particles.
2. Description of the Related Art
Oil and natural gas are produced from wells having porous and permeable subterranean formations. The porosity of the formation permits the formation to store oil and gas, and the permeability of the formation permits the oil or gas fluid to move through the formation. Sometimes the permeability of the formation holding the gas or oil is insufficient for economic recovery of oil and gas. In other cases, during operation of the well, the permeability of the formation drops to such an extent that further recovery becomes uneconomical. In such circumstances, it is common to fracture the formation and prop the fracture in an open condition by means of a proppant material or propping agent. Fracturing is usually accomplished by hydraulic pressure using a gel-like fluid. The pressure is increased until cracks form in the underground rock. The proppants, which are suspended in this pressurized fluid, are forced into the cracks or fissures. When the hydraulic pressure is reduced, the proppant material functions to prevent the formed fractures from closing again by “propping” the fractures open.
A wide variety of proppant materials are used, depending on the geological conditions. Typically, proppants are particulate materials, such as sand, glass beads, or ceramic pellets, which create a porous structure. Often, the proppants are coated with a resin to improve vital physical characteristics of the proppants. The oil or gas is able to flow through the interstices between the particles to collection regions, from which it is pumped to the surface. Over time, the pressure of the surrounding rock tends to crush the proppants. The resulting fines from this disintegration tend to migrate and plug the interstitial flow passages in the propped structure. These migratory fines drastically reduce the permeability, lowering the conductivity of the oil or gas. Conductivity is a measure of the deliverability or the ease with which oil or gas can flow through the proppant structure and is important to the productivity of a well. When the conductivity drops below a certain level, the fracturing process is repeated or the well is abandoned.
There are many physical characteristics of proppants that are important. Particle size of the proppant has a significant impact on the permeability, and resulting ability for hydrocarbon flow through the fracture, of the proppant pack. Crush strength of the proppant is another vital physical characteristic of the proppant because the proppant is subjected to high pressure levels as they prop open the fracture. Early proppants were formed of materials such as sand, glass beads, walnut shells, and aluminum pellets. However, where closure pressures of the fracture exceed a few thousand pounds per square inch these materials are crushed resulting in a closure of the fracture. In response, proppants having high compressive strength have been designed to resist crushing under the high pressure levels experienced in use. The crush strength of the proppants is related to the composition and density of the proppant material. Another important physical characteristic of the proppant is the shape of the individual particle, wherein roundness and a high level of sphericity are important characteristics.
The importance of the physical characteristics of proppants is well recognized in the industry. The American Petroleum Institute (API) has issued Recommended Practices for proppant testing. For example, API Recommended Practices RP-56 covers testing procedures for sand used in hydraulic fracturing operations. RP-58 provides testing procedure for sand used in gravel packing operations. RP-60 provides testing procedures for high-strength proppants used in hydraulic fracturing operations. These Recommended Practices include testing procedures for determination of properties that include, inter alia, particle size, crush resistance and sphericity and roundness.
Correct sampling technique of the proppants while gathering representative samples for testing is critical. If an improper sampling technique is used, a sample of the proppant gathered for laboratory analysis may not be representative of the entire proppant population being tested and the laboratory results will not provide the true physical characteristics of the proppant. The API Recommended Practices RP-56, RP-58 and RP-60 all include instructions for sampling proppants from the source of supply that include obtaining the samples by sweeping a collection device across the entire delivery stream as the proppants fall from a conveyer belt into a blender or other destination. Grabbing a sampling from a loaded silo or hopper does not provide a representative sample of the proppant.
Typically, proppants are delivered in bulk to the drilling site by trucks or railcar and unloaded pneumatically from the transport to a silo or hopper for storage until needed for injection during the fracturing procedure. When needed, the proppants flow by gravity from the silo to a mixer or blender to produce the fracturing liquid for injection into the reservoir. Unfortunately, the representative samples of the proppant that are tested are collected by using the sweeping collection device just as the proppant falls into the blender on its way to be injected as part of a well fracturing analysis. By the time the laboratory analyses are run on these representative samples of proppants at the drilling site, the proppants have already been injected into the reservoir. While the laboratory results of the proppant sample caught just before the proppant was mixed into the fracturing liquid document that a problem with the proppant existed, the results are too late to correct the problem by replacing the poor quality proppant with new proppant having the required physical characteristics.
What is needed is a method and apparatus for collecting a representative sample of the bulk proppant before the proppant is needed for injection. It would be beneficial if the representative sample of the proppant could be collected as it was being delivered off the transport truck or railcar.